1. Field of the Invention
The invention concerns a branched power transmission with several transmission ratio ranges and with a steplessly adjustable transmission ratio, as well as a parallel gearbox, especially for use in such a transmission. The invention further concerns a method for operating a branched power transmission. Additionally, the invention concerns a method for controlling the contact pressure or the contact force between the conical disks and the endless torque-transmitting means of a variable speed unit in such a branched power transmission. The invention further concerns such a transmission that additionally contains an electrical machine. Furthermore, the invention also concerns a method for the low-loss production of an output torque of the transmission when a motor vehicle is stationary as well as a method for operating a transmission with a steplessly variable transmission ratio and an electrical machine.
2. Description of the Related Art
Automatic transmissions with continuously adjustable transmission ratios are gaining increasing interest in motor vehicles not only because of the high driving comfort, but also because of the lower fuel consumption in comparison to conventional automatic transmissions that operate with planetary gear sets. Such transmissions contain, for example, a variable speed unit that is formed by two conical disk pairs enwrapped by an endless torque-transmitting means, wherein the spacing between the conical disks of the conical disk pairs is adjustable in opposite directions for adjusting the transmission ratio.
One problem with variable speed units lies in their limited transmission ratio adjustment range as well as in their limited torque transmitting capability. In order to enlarge the gear spread, meaning the transmission ratio adjustment range, and the torque transmitting capability, branched power automatic transmissions were produced in which the variable speed unit can be connected in various ways with a gear transmission through at least one clutch, wherein the adjustment range of the variable speed unit traverses is carried out during the change of the transmission ratio of the overall transmission in one direction or the other in accordance with the clutch position, so that an enlarged gear spread results in that case by a reduced variable speed unit spread. Further, at least in a branched power transmission in which a part of the drive torque is transmitted parallel to the variable speed unit through a clutch directly to the gear transmission or the output, the variable speed unit does not have to transmit the entire drive torque, whereby the torque transmitting capability of the transmission is enlarged.
FIG. 1 shows the basic structure of a first embodiment of a known branched power transmission.
The drive torque originating from an engine that is not shown and that is transmitted by a input shaft 2 is transmitted on the one hand directly to a variable speed unit 4 and on the other hand to a gear transmission 8 constructed as a summing transmission or a coupling transmission through a clutch 6, of which one input is connected with the output of the variable speed unit 4 and whose output is constituted by an output shaft 10, by which the motor vehicle is driven. One characteristic of the branched power transmission with the basic structure in accordance with FIG. 1 is that the variable speed unit 4 must transmit the full engine torque in the non-split region (clutch 6 disengaged) and must, therefore, be correspondingly designed for high torques and high rotational speeds, which means expense. Known branched power transmissions of that structure have a maximum spread of 7 and a torque transmitting capability of about 500 Nm.
A modified basic structure of a branched power transmission is shown in FIG. 2. In that structure, an input shaft 2 directly drives an input-side distributor transmission 12, for example a planetary transmission that has two outputs, one of which is connected with a variable speed unit 4 and the other is connected through a clutch 6 with the output shaft 10 which is non-rotatably engaged with the output of variable speed unit 4. Also in this embodiment, variable speed unit 4 must transmit the entire engine power when clutch 6 is disengaged.